Just eat alot of fruit/vegetables and stay away from alcohol and all types of meat. That is how you keep the fat away. Food doesn't affect your muscle, however.. simply upgrade physic to improve muscle

In order to see if a food is going to make you fat or not, push the left joystick on your controller to the right in the buy window. This should open up the rest of the food's stats. As mentioned above, make sure to eat all the celery you can lay your hands on and stop eating meats and pies. Good luck!

Celery is the only way you can lose wait.

Celery, it's the only thing that has a negative fat statistic, seriously.

Not entirely true... there is one more item that will reduce your weight by (if I remember correctly) -20%
Now this is if I can remember all this correectly, what I do know for sure is that it is the reward for "placing in a Spinnerbox Tourny" in the Pub Games.
I think it is the Amazing Apple Pie and Im pretty sure it is -20% to your fat. Or something rediculously huge like that.
Go to the Pub Game and go to the screen that shows you what the 15 rewards are. Its in the bottom Left Corner

Play spinnerbox tournement, the first one, the free one. When you get top 5, you win an apple pie. This lowers your fat by 90%. Eat it. I just did it the other day.

There are a few ways to lose weight, but the easiest is to eat all types of celery (they all have a positive impact on weight loss). Also, when you look at page 2 on the individual items screen it will tell you if an item will make you fat. I sold every item that had any fat content (the worst ones are meat and pies) and my Hero never had this problem.

Yes the only way (i know) to lose weight is to buy vegetable

And Also,If your character still looks fat after wolfing down loads of celery,it's his physique level.